1. Field of the Invention
The invention relates to multi-phase transformer design and more particularly to a three phase transformer arrangement which includes multiple secondary coils to allow balancing of the primary currents when unbalanced loads are present.
2. Description of the Related Art
Electrical power distribution and transmission have a number of problems. This is particularly true when three-phase distribution is being performed. As many simple loads are single phase loads, it then becomes necessary to balance the loads in a three phase system so that one particular phase is not overloaded. The overloading can require additional transformer sizing, wire sizing and so on to handle the additional currents, and can be destructive to the various equipment if not properly sized. If properly sized to handle the imbalance, this results in more expensive components and unused capacity in the lightly loaded phases.
So while load balancing is highly desirable, in many cases this is very difficult to perform. For instance, in a distribution system loads are commonly switched on and off, so that while under maximum load conditions a balanced or near balanced condition could be present, under less than full load conditions where certain loads are on and certain loads are off, an imbalance can readily develop. As this imbalance develops, then of course the primary currents are unbalanced and this trickles up through the entire system, causing problems.
Various attempts have been made to develop systems to automatically balance the loads but in most cases this required an active component of some sort. The most common way is to perform tap changing under load, where the turns ratio of the transformer is changed, changing the voltage of a phase and hence its current. Of course this greatly complicates transformer designs and requires the mechanical mechanisms needed to actually set new taps. As a result, the durability of the device is reduced, maintenance requirements are increased and cost is dramatically increased. Various electronic means have been tried but these are not readily acceptable in a power distribution environment, only for small loads. Further, the electronics adds additional costs and maintenance requirements.
Thus the current solutions have many drawbacks. It is desirable to have a simple passive transformer design wherein load balancing is achieved without the use the any active components such as tap changers or electronics, and without unduly increasing the cost and complexity of the transformer.